Derive Jason.Encoder for generated State struct

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Generated `State` structs now derive `Jason.Encoder`, fixing `Protocol.UndefinedError` when encoding state over Phoenix channels or other JSON serialization paths
+
 ## [0.3.0] - 2026-02-26
 
 ### Fixed

lib/durable_object/dsl/transformers/build_introspection.ex

@@ -59,6 +59,7 @@ defmodule DurableObject.Dsl.Transformers.BuildIntrospection do
         quote do
           defmodule State do
             @moduledoc false
+            @derive Jason.Encoder
             defstruct unquote(Macro.escape(struct_fields))
           end
         end

test/durable_object/dsl/transformers_test.exs

@@ -78,5 +78,25 @@ defmodule DurableObject.Dsl.TransformersTest do
       get_participants = Enum.find(handlers, &(&1.name == :get_participants))
       assert get_participants.args == []
     end
+
+    test "State struct is JSON-encodable via Jason" do
+      state = %BasicCounter.State{id: "test-123", count: 42}
+      assert {:ok, json} = Jason.encode(state)
+      assert %{"id" => "test-123", "count" => 42} = Jason.decode!(json)
+    end
+
+    test "State struct with complex fields is JSON-encodable" do
+      state = %ChatRoom.State{
+        id: "room-1",
+        messages: [%{user: "alice", text: "hello"}],
+        participants: ["alice", "bob"],
+        created_at: nil
+      }
+
+      assert {:ok, json} = Jason.encode(state)
+      decoded = Jason.decode!(json)
+      assert decoded["id"] == "room-1"
+      assert length(decoded["participants"]) == 2
+    end
   end
 end

test/durable_object/server_persistence_test.exs

@@ -100,6 +100,42 @@ defmodule DurableObject.ServerPersistenceTest do
     end
   end
 
+  defmodule SerdeCounter do
+    use DurableObject
+
+    state do
+      field(:count, :integer, default: 0)
+    end
+
+    handlers do
+      handler(:increment)
+      handler(:get_state)
+    end
+
+    def handle_increment(state) do
+      {:reply, :ok, %{state | count: state.count + 1}}
+    end
+
+    def handle_get_state(state) do
+      {:reply, state, state}
+    end
+  end
+
+  describe "state serde through sqlite" do
+    test "state persists and rehydrates correctly" do
+      id = unique_id("serde")
+
+      {:ok, :ok} = SerdeCounter.increment(id, repo: TestRepo)
+      {:ok, :ok} = SerdeCounter.increment(id, repo: TestRepo)
+      {:ok, :ok} = SerdeCounter.increment(id, repo: TestRepo)
+      DurableObject.stop(SerdeCounter, id)
+
+      {:ok, state} = SerdeCounter.get_state(id, repo: TestRepo)
+      assert state.id == id
+      assert state.count == 3
+    end
+  end
+
   describe "without :repo option" do
     test "works without persistence" do
       id = unique_id("no-repo")